disp: msm: sde: avoid wb commit with null output framebuffer

Writeback commit with null output framebuffer is
not a valid usecase. Add support to fail such
scenarios.

Change-Id: I78f89908d95e1f28218491efb9d1d5df3b7608ff
Signed-off-by: Krishna Manikandan <mkrishn@codeaurora.org>